sync: cleanup sleep+retry logic a bit

Make sure we print a message whenever we retry so it's clear to the
user why repo is pausing for a long time, and why repo might have
passed even though it displayed some errors earlier.

Also unify the sleep logic so we don't have two independent methods.
This makes it easier to reason about.

Also don't sleep if we're in the last iteration of the for loop.  It
doesn't make sense to and needlessly slows things down when there are
real errors.

Bug: https://crbug.com/gerrit/12494
Change-Id: Ifceace5b2dde75c2dac39ea5388527dd37376336
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/303402
Reviewed-by: Sam Saccone 🐐

 <samccone@google.com>
Tested-by: Mike Frysinger <vapier@google.com>